The stress system relies on two key hormones: adrenaline and cortisol. In short, adrenaline works in the short term, while cortisol has large momentum and works in the long term. (Adrenaline is also known as epinephrine in North America. To be exact, the terms noradrenaline and norepinephrine are used to refer specifically to the neurotransmitter as opposed to the hormone, since they are different molecules. Moreover, there are many other neurotransmitters involved: check reference [1] for details. The purists will excuse my exclusive use of the word adrenaline throughout the text).

It is important to realise that the stress system can also be activated if your brain perceives danger or any kind of threat. In the first stage, this triggers the release of adrenaline into the bloodstream to prepare the body for action. As a result, your heart beats faster, you begin to sweat, your breath becomes shallower, and your senses become more acute. This is the so-called fight or flight response to the stressor event, and was quite adequate during most of our evolution, when these events were quite specific and usually short-term: escaping from a lion, chasing away a rival gang, or facing up to the impudent adolescent trying to woo your mate. Problems with chronic stress arise because in a modern society we cannot escape easily from the stressor, be it an overbearing boss, crowded cities, or traffic jams. Furthermore, no matter how hard we try to delude ourselves with the pretence of civilisation, at heart we are still primates, and consequently, factors such as social status also play an important role as sources of stress. Moreover, primates have evolved the capacity to stress up the body in anticipation of a possible danger [2]. Again, this was an advantageous adaptation in the context where it evolved, but nothing but trouble for the modern human.

The effect of the stress hormones on the brain is curious and not what you might expect. The initial surge of adrenaline can make you feel good.

Figure 2: The relationship between adrenaline and cortisol.

Just as your levels of adrenaline start coming down, so rises the amount of cortisol flowing through your veins. Moreover, cortisol has a much larger momentum than adrenaline, which means that even though it builds up slowly, it also takes a long time to go back to normal. And should you constantly be engaging in activities which require adrenaline, so will your levels of cortisol slowly increase. In a sense, you can think of cortisol as a measure of the weighted average of your recent levels of adrenaline. I have tried to capture this feature in Figure 2.

Together with the rise of cortisol and the decrease of adrenaline, come the nasty side-effects of the stress hormones. It is at this moment that you feel bad, anxious, and having lots of negative thoughts. And this is perhaps one of the critical features of stress which flies against common sense: you only feel its bad aspects when your body is stressing down and progressing towards a more relaxed state. When you are building up on adrenaline, in effect stressing up, you might even be feeling good! This explains what is popularly known as the adrenaline rush and the consequent adrenaline crash.

Having too much cortisol flowing through your veins has another nasty side-effect: the recovery time from any adrenaline surge increases. In a sense, the relation between adrenaline and cortisol goes both ways: the adrenaline curve influences the cortisol curve, and vice-versa. Figure 3 tries to capture this reaction effect by showing the adrenaline response curve for three individuals subjected to the same physical exercise. Notice how the more serious the depression (which translates into higher levels of cortisol, as you will soon understand), the longer it takes for the body to go back to normal.

Figure 3: The adrenaline response curve for various degrees of depression.

On the speculation front, recent findings may have implicated neuron death as the physical underpinning of depression. Furthermore, it seems that the opposite process, termed neurogenesis, is crucial for the recovery, and happens naturally in healthy individuals [3,6]. Furthermore, evidence indicates that sleep is fundamental for neurogenesis to take place.